1. Field of the Invention
The present invention relates to a semiconductor laser device capable of operating at high efficiency and high output power, which is preferably used in the fields of communication, printing, laser medical treatment, laser beam machining and the like.
2. Description of the Related Art
For the purpose of enhancing an output power of a semiconductor laser, the applicant of the present application has proposed a semiconductor laser which is provided with carrier blocking layers having a wide band gap and a small thickness on both sides of an active layer, whereby the design freedom of a band gap of a cladding layer formed outside the carrier blocking layer is increased (WO 93/16513).
In such a configuration, the carrier blocking layer has a function of confining an injected carrier in the active layer efficiently and the carrier blocking layer is formed into a thin shape, so that light generated in the active layer can pass through the carrier blocking layer and leak out easily to an optical guide layer which is disposed outside. Therefore, it is possible to avoid catastrophic optical damage which occurs due to a localization of laser light on an emission facet of a semiconductor laser, and raise the breakdown level of a facet, with the result that an operation at high output power can be realized.
In order to fabricate a semiconductor laser device of higher efficiency, it is important to decrease losses, among which an inner loss depends on free carrier absorption to a large extent. In this free carrier absorption, a p-type layer is more involved than an n-type layer. In the case of using GaAs, for example, the free carrier absorption coefficient .alpha..sub.fc [cm-.sup.1 ] is expressed by formula (1) as shown below (see page 85, "Semiconductor Laser--basis and application--," edited by Ryoichi ITO and Michiharu NAKAMURA): EQU .alpha..sub.fc =3.times.10.sup.-18.multidot.n+7.times.10.sup.-18.multidot.p (1)
wherein n denotes the concentration of an n-type carrier and p denotes the concentration of a p-type carrier. It is apparent from formula (1) that the free carrier absorption coefficient afc is proportional to the concentration of carriers and the p-type layer is involved in free carrier absorption twice as much as the n-type layer or more. PA1 a first cladding layer, PA1 a first optical guide layer, PA1 a first carrier block layer, PA1 an active layer, PA1 a second carrier blocking layer, PA1 a second optical guide layer, PA1 a second cladding layer, PA1 band gaps of the first and second optical guide layers being wider than that of the active layer, PA1 band gaps of the first and second cladding layers being wider than those of the first and second optical guide layers, PA1 band gaps of the first and second carrier blocking layers being wider than those of the first and second optical guide layers, PA1 one of the first and second cladding layers being of p-type, the other being of n-type, PA1 wherein a refractive index of the p-type cladding layer is lower than that of the n-type cladding layer.
In order to fabricate a semiconductor laser device of further higher efficiency and higher output power, it is important to limit the electric resistance and thermal resistance of the device to a low level. When the electric resistance is high, the energy conversion efficiency is decreased due to the generation of Joule's heat and the like. Moreover, since the temperature of the device rises, a threshold current is increased and an output power is decreased due to heat saturation.